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'and to the distal, in which the decay has not yet reached the
dentine.
In the preparation of buccal and labial cavities for filling
the requirement is that all of this area of uncleanliness be
removed at once, including, of course, the last traces of soft-
;€ned enamel, no matter how sound and perfect the dentine
may be beneath, and place the occlusal line of the cavity mar-
gin so far to the occlusal that it will be kept clean by masti-
cation of food, the gingival line so that it will be well covered
by the gum margin, and the mesial and distal lines at the
most favorable points near the mesial and distal angles of
the tooth. The object is the removal of the area of liability
to decay for the prevention of its recurrence. Wherever the
susceptibility to this class of decay is considerable, nothing
less will effect a cure. If in these cases the cavity in the
dentine, simply, is excavated and filled ever so well, decay will
go on to the mesial and to the distal of the filling so quickly
'•that it will be of no value whatever. Formerly these were
regarded as the most uncertain of fillings, because of this
speedy recurrence of the decay, but since the principles of
extension for prevention, as above detailed, have been used
in their treatment, these fillings have become almost as
certain of long endurance as fillings in occlusal cavities. The
main point in their successful treatment is the careful study
of the local conditions, the condition of susceptibility being
appreciated, and so cutting the cavity as to meet these con-
ditions. This becomes a matter of judgment in each indi-
vidual case, and upon this judgment success or failure de-
pends. It is always safer to cut wider than necessary than
to fall a little short.
The greatest difficulties are met with when these decays
occur in young persons. In this case the teeth are more
apt to be hypersensitive and the patient difficult to control,
and great patience, persistence and moral courage are re-
quired of the operator to enable him to carry the cutting to
the proper limit. At the same time, the requirements for ex-
tension of the cavity are much greater than in older persons.
The susceptibility to decay is greater. More time must elapse
before there is relief by the coming of immunity, and for these
reasons more is required of the filling. Also the free mar-
gin of the gum is long in young persons and becomes shorter

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